The present invention relates to a method of patterning an organic polymer film and a method for fabricating a semi-conductor device by utilizing the patterning method.
Recently, millimeter wave bands, or ultrahigh frequency bands of 30 GHZ or more, are frequency resources that should hopefully be developed for a broad variety of applications including multi-media mobile telecommunications units and radio frequency LANs. To ensure a sufficient gain for an FET in ultrahigh frequency bands like these, the gate electrode of the FET should have its length and resistance both reduced. On top of that, a parasitic capacitance associated with the gate electrode should also be reduced. A T-shaped (or mushroom-shaped) gate electrode would be a best choice so far among various measures for reducing the gate length and gate resistance. So a T-gate electrode is adopted more and more often for that purpose.
A typical known semiconductor device including a T-gate electrode will now be described with reference to FIG. 3, which schematically illustrates a cross-sectional structure for a known semiconductor device of this type.
As shown in FIG. 3, the device includes semi-insulating GaAs substrate 51, epitaxial layer 52 deposited on the substrate 51 and a T-gate electrode 56 formed on the epitaxial layer 52. The bottom of the T-gate electrode 56 makes a Schottky contact with the surface of the epitaxial layer 52. A pair of ohmic electrodes 55 is further formed on, and makes an ohmic contact with, the epitaxial layer 52. For the other parts that are not covered with the T-gate electrode 56 or ohmic electrodes 55, the upper surface of the epitaxial layer 52 is covered with an interlevel dielectric film 54 of SiO.sub.2. Also, to electrically isolate the illustrated device from adjacent ones, the epitaxial layer 52 is surrounded with an isolation region 53.
A method for fabricating the known semiconductor device will be described next with reference to FIGS. 4A through 4G, which illustrate respective process steps for fabricating the device shown in FIG. 3.
First, as shown in FIG. 4A, an epitaxial layer 52 is deposited on a semi-insulating GaAs substrate 51 by an MOCVD or MBE process, and an isolation region 53 is defined by implanting dopant ions into a selected region of the substrate.
Next, as shown in FIG. 4B, an insulating film 54 of SiO.sub.2 is deposited on the epitaxial layer 52 by a CVD process, and then a photoresist 55, having an opening 55a with a width of 0.1 .mu.m, is defined on the insulating film 54.
Thereafter, as shown in FIG. 4C, an opening 54a is formed in the insulating film 54 by dry-etching the film 54 anisotropically using the photoresist 55 as a mask, and then the photoresist 55 is removed as shown in FIG. 4D.
Subsequently, as shown in FIG. 4E, parts of the insulating film 54, where ohmic electrodes will be formed, are removed to form another pair of openings, and then ohmic electrodes 56 are formed on the particular areas of the epitaxial layer 52 that are exposed inside the openings. Next, another photoresist 57 with an opening 57a is defined as shown in FIG. 4F.
Finally, a metal film (not shown) is deposited over the photoresist 57 so that the opening 57a is filled with the metal, and then the photoresist 57 is removed along with the excessive metal, thereby forming a T-gate electrode 58 as shown in FIG. 4G.
This device includes the T-gate electrode 58, and can have a shorter gate length and reduced gate resistance. However, the insulating film 54 is made of SiO.sub.2 with a dielectric constant of about 4.0, so the gate parasitic capacitance is not so small. That is to say, this device has a large fringe capacitance due to the particular shape of the gate electrode 58 and the material of the insulating film 54.
To reduce the fringe capacitance of the gate electrode 58, the insulating film 54 should preferably be made of a material with a lower dielectric constant (which will be herein called a "low-.kappa. material"). An organic polymer may be used as an alternative material for the insulating film 54, because an organic polymer has a dielectric constant lower than that of SiO.sub.2. However, if the above process is performed as it is just by substituting an organic polymer for SiO.sub.2, then it is difficult to form the opening 54a at a desired small size.
In the above process, the opening 54a is formed in the insulating film 54 by dry-etching the film 54 anisotropically using the photoresist 55 with the opening 55a as a mask as shown in FIG. 4C. Then, the opening 54a of the insulating film 54 will usually be greater in width than the counterpart 55a of the photoresist 55. This is also true even when the insulating film 54 is made of an organic polymer. In that case, the width of the resultant opening 54a will be no less than about 0.7 .mu.m, for example. That is to say, the opening 54a cannot have a width as small as 0.3 .mu.m or less (e.g., 0.1 .mu.m) according to the known process.
To avoid this problem, the opening 54a may be formed by a lift-off technique, not by using the photoresist 55 having the opening 55a. But we found that another problem is caused by doing so.
FIGS. 5A and 5B are cross-sectional views illustrating the process steps of forming an opening by a lift-off technique. First, a substrate 61 is prepared, and a fine-line resist pattern 62 is defined on the substrate 61 by a photolithographic technique as shown in FIG. 5A. Next, as shown in FIG. 5B, an organic polymer film 63 is deposited over the substrate 61. However, since an organic polymer is usually liquid, the resist pattern 62 cannot be lifted off as it is. That is to say, even if the resist pattern 62 is lifted off, the film 63 of the liquid organic polymer will planarize itself after that. As a result, no opening can be formed in the organic polymer film 63. To form an opening in the organic polymer film 63 by a lift-off technique, the liquid organic polymer should be cured by annealing it at 200.degree. C. or more. However, the resist pattern 62 is usually cured or deformed at about 150.degree. C. Accordingly, it is meaningless to cure the liquid organic polymer by annealing it at 200.degree. C. or more.